1. Field of the Invention
The present invention relates to a step-up circuit and a step-up circuit device.
2. Description of the Related Art
A step-up circuit is a circuit configured to convert (step up) a voltage of a voltage source which is applied to a power supply terminal into a step-up voltage higher than the voltage of the voltage source and output the step-up voltage, and is built into various electronic devices. A step-up method in a step-up circuit includes a fly-back method using transformers, a charge pump method using capacitors, etc. The charge pump method can achieve a smaller-size because of the absence of the transformers as compared to the fly-back method. In particular, the charge pump method is widely used in portable devices which are required to be smaller-sized and thinned.
FIG. 10 is a view showing a configuration of a conventional step-up circuit device using the charge pump step-up circuit (see Japanese Laid-Open Patent Application Publication No. 2007-300760, especially FIG. 2). As shown in FIG. 10, a step-up circuit device 2 includes a charge pump step-up circuit 6 configured to step up a voltage source voltage VCC in such a manner that, using diodes DA and DB as charge transfer switches, charge is stored in a capacitor CA and the charge is transferred from the capacitor CA to a capacitor CB repetitively, based on a clock signal.
The step-up circuit 6 is implemented as a semiconductor integrated circuit, and includes a terminal T1 (power supply terminal) to which the voltage source voltage VCC is applied, a terminal T2 (output terminal) through which a step-up voltage VG is output, terminals CP1 and CP2 (capacitor connecting terminals) for connecting the electrodes of the capacitor CA, etc. The capacitor CB is connected to the terminal T1 and the terminal T2.
The step-up circuit 6 includes a step-up section 9 comprising the diodes DA and DB and a level shift circuit 7. The diodes DA and DB are connected in series between the terminal T1 and the terminal T2. The anode of the diode DB is connected to the terminal CP2, while the cathode of the diode DB is connected to the terminal T2. The step-up section 9 and the capacitors CA and CB constitute a charge pump circuit. The level shift circuit 7 receives as an input a clock signal CLK (H level: V0 (V), L level: 0(V)), shifts a level of the clock signal CLK and outputs a clock signal CLK1 (H level: VREG (V), L level: 0(V)). The clock signal CLK1 is input to the terminal CP1.
The step-up circuit 6 performs a step-up operation in such a manner that, using the clock signal CLK1 (clock signal), the diodes DA and DB are complementarily turned ON and OFF, charge is stored in the capacitor CA and the charge stored in the capacitor CA is transferred to the capacitor CB, which occurs repetitively. As a result of the step-up operation, a voltage at the terminal T2 rises from 0(V) to “VCC−2×Vd+VREG(V).” That is, the step-up circuit 6 steps-up the voltage source voltage VCC applied to the terminal T1 to the step-up voltage VG of “VCC−2×Vd+VREG(V)” and outputs the step-up voltage VG through the terminal T2.
If the terminal CP2 is shorted to ground, a ground-fault current flows through a current path including the terminal T1, the diode DA, and the terminal CP2, which possibly causes the diode DA to be broken down. If the terminal T2 is shorted to ground, a ground-fault current flows through a current path including the terminal T1, the diode DA, the diode DB and the terminal T2, which possibly causes the diode DA and the diode DB to be broken down. To avoid this, the step-circuit 6 has a ground-shorting protective function, for preventing an excess ground-fault current from flowing through the diode DA or the diode DA and the diode DB, which would damage the diode DA or the diode DA and the diode DB.
A switch SW and a switch control circuit 8 performs the above ground-shorting protective function. The switch SW is provided between the terminal T1 and the anode of the diode DA and is placed in a conductive state (normal state) or in a non-conductive state (shorted-to-ground state) based on a switch control signal CTRL from the switch control circuit 8. The switch control circuit 8 receives as inputs the voltage source voltage VCC applied to the terminal T1, a diode voltage VD1 corresponding to the voltage at the terminal CP2, and the step-up voltage VG corresponding to the voltage at the terminal T2.
The switch control circuit 8 detects whether or not the step-up voltage VG is below a predetermined threshold voltage. When detecting that the step-up voltage VG is below the predetermined threshold voltage, the switch control circuit 8 detects that the terminal CP2 or the terminal T2 is shorted to ground. Then, the switch control circuit 8 outputs the switch control signal CTRL to switch the switch SW from the conductive state to the non-conductive state. As a result, the current path through which the above ground-fault current flows is disconnected, thereby preventing the ground-fault current from flowing through the diode DA and the diode DB.
As shown in FIG. 5 of the above publication, the switch control circuit 8 includes a detecting section 8A constituted by a resistance voltage divider circuit, a comparator, etc, and a timer latch circuit 8B constituted by a timer circuit, a latch circuit, etc. Thus, since circuits other than the above step-up section are required to perform the ground-shorting protective function, a problem will arise, in which a circuit scale of the step-up circuit increases and reduction of the size and cost of the step-up circuit device is not fulfilled. Furthermore, at least the resistance voltage divider circuit and the comparator consume a control current. This increases current consumption in the step-up circuit device.